1. Field of Invention
The present invention relates to an optical recording medium, an optical recording and reproducing method, and an apparatus that realizes high density and high speed in data recording and data transferring, respectively.
2. Description of Related Art
In optical recording, rewritable optical disk apparatus such as a phase-change type and a magneto-optical disk type have heretofore been widely used for high-density recording. Although these optical disk apparatus are higher than ordinary magnetic disk apparatus in recording density by more than one order of magnitude, such recording density is not yet useful for recording digital image data. In order to further increase a recording density, it is necessary to reduce a distance between adjacent tracks or adjacent bits by reducing a diameter of a beam spot.
A DVD-ROM that uses such optical recording technology is commercially available. The DVD-ROM is a 12-cm disk which is able to store 4.7 gigabytes of data on one side. A rewritable DVD-RAM is able to record 5.2 gigabytes of data with a high density on both sides of a 12-cm disk and uses phase-change recording. This disk is able to write and read information and has a storage capacity that is more than 7 times that of a read-only CD-ROM and that is equivalent to more than 3600 floppy disks. As described above, technology for increasing the recording density of optical disks is being advanced every year. However, on the other hand, since the above-mentioned optical disk records data within the surface, the optical disk is limited by diffraction so that a recording density of approximately 5 gigabits/inch2 is the physical limit of high-density optical recording.
Accordingly, in order to realize the higher density/speed of data recording and transferring, a method of recording multilevel data in one recording pit is proposed. Such technologies are described in Japanese Published Unexamined Patent Application No. Sho 64-17248 and Japanese Published Unexamined Patent Application No. Hei 4-38720. A fundamental principle of magneto-optical recording is to read out a change in polarization angle of reflected light caused by a magneto-optical Kerr effect in the recording area. However, although there are provided various devices, the change of the polarization angle obtained by the Kerr effect is very small, e.g. about 1 degree. As a result, it is difficult to provide multilevel data within this small polarization angle change from a Signal-to-Noise ratio (S/N) standpoint. Therefore, there are made various attempts to widen the dynamic range of the signal.
The technology described in Japanese Published Unexamined Patent Application No. Sho 64-17248 uses a multilevel recording medium formed of multiple layers of recording films having different Curie temperatures. Also, the technology described in Japanese Published Unexamined Patent Application No. Hei 4-38720 uses elliptic recording pits to record and reproduce multilevel data on and from the optical disk and provides multilevel data by changing an inclination amount of elliptic recording pits.
However, according to the typical manufacturing process for making a multilayer recording film, the number of the layers that can be laminated to provide the multilayer structure is limited, and the dynamic range cannot be widened. Therefore, much of the multilevel-modulation cannot be obtained. Also, there is then the problem that, when the film thickness of the recorded portion increases as the number of the layers in the multilayer increases, a recording density in the surface direction is lowered due to the diffusion of heat into the disk surface. On the other hand, according to the method in which the recording pits are formed as elliptic recording pits, in order to keep a recording density similar to that of the conventional method, it is unavoidable that the area of the recording pits is decreased as compared with the ordinary case because the recording pits are formed in an elliptical shape. As a result, it is not possible to avoid the signal intensity from being lowered in the single substance of the recording pit so that a deterioration in the S/N ratio occurs. Further, with respect to signal detection, a difference produced between the reproduced light patterns due to the inclination angle of the pits is detected by a plurality of detectors. If the amount of multilevel information is increased, then a difference between the patterns of each respective gradation becomes considerably small so that the signal at each detector becomes very small. Therefore, if information is stored as multilevel information having a large number of different gradation levels, a lack of detection accuracy occurs. Hence, multilevel information having a large number of gradation levels cannot be obtained.